Various characteristics such as heat resistance, heat-resistant dimensional stability, moisture-resistant dimensional stability, electric insulating property, deformation resistance (hardly causing torsion, curvature, waving, or the like), and lightness are required for substrates used for laminates for electric circuits.
Since seat-resistant fiber paper sheets are excellent at points such as heat resistance, electric insulation, heat-resistant dimensional stability and lightness in comparison with those of other raw materials, the heat-resistant fiber paper sheets are recently being used for the substrates of laminates for electric circuits.
For example, a paper sheet [JP-A 2-236907 (hereunder, JP-A means “Japanese Unexamined Patent Publication”), JP-A 4-6708] comprising poly-meta-phenylene isophthalamidc staple fibers ([CONEX®], produced by TEIJIN LTD.) and poly-meta-phenylene isophthalamide pulp, a heat-resistant fiber paper sheet (JP-A 1-92233, JP-A 2-47392) comprising copoly-para-phenylene.3,4′-oxydiphenylene.terephthalamide fibers ([TECHNORA®], produced by TEIJIN LTD.) and an organic resin binder, and the like, have been proposed.
However, the former not only shrinks when thermally treated at a high temperature of not less than 250° C., but is also especially inferior in an electrical insulating property in a high humidity, because having a large fiber equilibrium moisture content (water content) of 5 to 6% and a large impure ion content. Thereby, the former can not be used for electrical insulating substrates for which high reliability is required.
On the other hand, although having a small equilibrium moisture content and a small impure ion content, the latter has a problem that uniformity and reliability in the thickness direction of the paper sheet are deteriorated, because an organic resin used as a binder component is unevenly migrated on the front and back sides of the paper sheet in a process for making the paper sheet to reduce the content of the binder component in the middle layer portion of the paper sheet.
When such the heat-resistant fiber paper sheet is used as a substrate of a laminate for an electric circuit board, troubles have often occurred such as the breakage of the paper substrate, the mutual movement of the paper-forming staple fibers to deteriorate the uniformity of the fiber density distribution, and the deformation (torsion, curvature, waving or the like) of the laminate for the electric circuit board, for example, after the finish of a solder reflow process in which the fiber paper sheet is especially treated at a high temperature, because the expansion in the unevenness of the impregnation (especially in the thickness direction) and adhesion of a mixed varnish and the deterioration in adhesive forces between the fibers due to the partial melting of the binder resin are caused in a process for producing the laminate, especially in a process for making a prepreg by impregnating the heat-resistant fiber sheet with the mixed varnish such as an epoxy resin and then drying the impregnation product, a process for laminating and shaping the said prepreg, and the like. Thereby, the heat-resistant fiber paper sheet is not preferable.
Further, a paper sheet [JP-A 61-160500, JP-B (hereunder, JP-B means “Japanese Examined Patent Publication”) 5-65640] has also been proposed in which para-aromatic polyamide staple fibers ([KEVLAR®D], produced by DU PONT LTD.) and fibrillated para-aromatic polyamide fine fibers ([KEVLAR®], produced by DU PONT LTD.) are mechanically bound to each other by the interlacing action of meta-aromatic polyamide fibrids used instead of the organic resin as the binder component.
This paper sheet has excellent characteristics such as excellent heat resistant, excellent heat-resistant dimensional stability, excellent moisture-resistant dimensional stability and excellent deformation resistance (hardly causing torsion, curvature, waving, or the like), but the binder component used in the paper sheet is only the fibrids. Therefore, in order to maintain a necessary tensile strength in each process for producing laminates for electric circuit boards, the addition amount of the fibrids must have been increased on the practical production. However, since the fibrids used as a binder component in the paper sheet and comprising the meta-aromatic polyamide have a high equilibrium moisture content and a large impure ion content, the paper sheet has had a problem that the paper sheet is very often judged to be defective in an electric insulation test carried out in a high humidity.
Namely, when a laminate used for an electric circuit board and formed using the substrate having a high percent of water absorption (equilibrium moisture content) in a large amount as a main material is energized in a high humidity for a long period, the contained impure ions are migrated to cause the failure of electric insulation. Therefore, the laminate can not maintain reliability for a long period.
Further, an aromatic polyamide fiber paper sheet (JP-A 9-228289) comprising staple fibers comprising a para-aromatic polyamide and staple fibers comprising a meta-aromatic polyamide has also been proposed, but since the meta-aromatic polyamide fibers are used, the paper sheet has a high percent of water absorption and a large impure ion content, and hence has a problem that the paper sheet is often judged to be defective in a substrate electric insulation test carried out at a high temperature, and a problem that an interlayer electric resistance value in the thickness direction of a printed circuit board is largely changed, when the absorption and desorption of moisture are repeated, as described above.
Although various kinds of the heat-resistant fiber paper sheets have been proposed as described above, a paper substrate has not been realized which has a low percent of water absorption, a low impure ion content, excellent electric insulation, excellent uniformity in the thickness of the paper sheet, good mixed varnish impermeability, excellent interlayer bondability, and excellent deformation resistance and is used for electrical insulating materials.